The situation is worsening, as expected. However the number of deaths is still very low, and life more or less goes on as normal at the moment.
But from tomorrow, nightlife has to be shut by 23:00 (I think, I've heard conflicting reports) and in the near future there will be strict restrictions on the number of people at indoor and outdoor events. You can check for the latest news here: https://spectator.sme.sk/c/22486654/czechia-may-be-added-to-the-red-list-and-more-new-rules-are-put-in-place.html
The bigger issues are whether you think you might have to go into isolation when you return from Slovakia - best to check with your home country about that.
It's a bit grim, Slovakia has always been on a bit more "panicky" side of things during the pandemic. However, most places are open normally and you can still have a decent trip if you don't mind omnipresent masking and some social distancing. I believe restaurants and bars can be open normally, even after 23:00, the curfew was only meant to apply to organised events (and even then I'm not sure if it actually passed).
Frankly, it's not a destination I'd choose these days but things are open and if you already booked your trip, there's aren't many reasons to cancel at this point.Edited: 13 September 2020, 18:57
My bad, restaurants and bars indeed have to close at 23:00, I finally found the decree. Well...
Basically we just want to get some walks in and have a change of scenery
Masks and social distancing is the norm for us
Then you will be absolutely fine :)
Lakes: in Bratislava you can swim in Drazdiak or Zlate Piesky. They are 'city lakes' but are both clean and have some attached amenities (bars, restaurants, toilets) and give a good taste of what locals do to relax. Don't expect unspoiled nature or anything like that though. But they will be nice and quiet during the week :)
There are some wilder lakes at Cunovo and Jarovce (note that it's forbidden to swim in the former, or the bigger lake there at least)
There are loads of lakes in Slovakia, but tourist season is ending and they will be cold and maybe won't have so may facilities out of season. The lakes around Banska Stiavnica are lovely (I've been to Klinger at the height of summer and many people were swimming) but not sure how it is in September.Edited: 13 September 2020, 23:10